ISSN: Original Article Synthesis, Structure and Photoluminescent properties of LaPO4:Er 3+ nanophosphor

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1 Available online at Nanoscience and Nanotechnology: An International Journal Universal Research Publications. All rights reserved ISSN: Original Article Synthesis, Structure and Photoluminescent properties of LaPO4:Er 3+ nanophosphor Niyaz Parvin shaik 1,*, N.V.Poornachandra Rao 2 and K.V.R.Murthy 3 1,* Department of Physics,Government Junior College, Sathupally, Khammam District, Telengana State ,India 2 Department of Physics, Rajiv Gandhi University Knowledge Technologies, Basara , India 3 Display Materials Laboratory, Applied Physics Department, Faculty of Technology &Engineering,M.S University of Baroda, Baroda , India *Corresponding author sk.niyaz@gmail.com Received 07 September 2014; accepted 23 September 2014 Abstract LaPO 4:Er 3+ (0.5 mol %) nanophosphor was synthesized by the conventional solid-state reaction method. X-ray diffraction (XRD), scanning electron microscopy (SEM),Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL) spectra and the particle size analysis were used to characterize the LaPO 4 :Er 3+ nanophosphor.the XRD results reveal that the synthesized LaPO 4:Er 3+ phosphor is well crystalline and assigned to the base centered monoclinic structure with a main (120) diffraction peak. The calculated crystallite size of LaPO 4:Er 3+ phosphor is 64.12nm. The excitation spectrum of LaPO 4:Er 3+ nanophosphor monitored under 400nm wavelength was characterized by a broad band ranging from nm with a maximum intensity peak at 246nm (5.04eV). Upon excitation at 254nm wavelength, the emission spectrum of LaPO 4:Er 3+ nanophosphor emits a broad band range from nm with maximum intensity peak at 469(blue) nm (2.64eV).The color coordinates for the LaPO 4:Er 3+ (0.5 mol %) nanophosphor are x= and y= This phosphor is having excellent colour tunability of blue light Universal Research Publications. All rights reserved Key words: Photoluminescence; XRD; FTIR; nanophosphor; rare-earth ions; solid state reaction method; CIE. 1. Introduction The optical properties of the lanthanides in inorganic compounds, their preferred valence, and their electron donating or accepting properties are all determined by the electronic structure, i.e., by the relative and absolute level energies of the lanthanide impurity states and the host conduction and upper valence band states. Rare-earth orthophosphates (REPO 4) are a very interesting class of host lattices of activator ions due to their physic-chemical inercy (high insolubility, high thermal stability), thus providing durable phosphors [1]. Lanthanide ortho- -phosphate (LnPO 4) belongs to two polymorphic types, the monoclinic monazite type (for La to Gd) and quadratic xenotime type (for Tb to Lu).The LaPO 4 crystalline matrix is widely used for development of phosphors for compact fluorescent lamps,plasma display panels, field emission display, optical amplifiers, laser active mediums and electrical conduction. Process ability of this material and resistance for atmospheric influence caused the interest to study of the luminescent properties of LaPO 4 nanoparticles. Monoclinic lanthanum phosphate is a compound with extremely low solubility in water,and high thermal and chemical stability; it has been proposed for use in broad applications.the luminescent properties of rare-earth phosphates can be conferred by the presence of lanthanide(iii) ions as activators due to their intense and narrow emission bands arising from f-f transitions, which are proper for the generation of individual colours in multiphosphor devices [2-4]. Lanthanide orthophosphate (LaPO 4) and lanthanide (III)-doped lanthanide orthophosphate have attracted much attention due to their unique photoluminescent properties and various potential applications in various areas, including color displays, light sources, field-effect transistors, solar cells, and biomedical labels,nanoscintillators for radiotherapy [5,6].It is known that the LaPO 4 has a monoclinic phase of monazite structure crystallographically,wherein La 3+ ion is nine coordinated to oxygen atoms, four oxygens forming a distorted tetrahedron interpenetrating a quasiplanar pentagon formed by another five [7-10]. The La 3+ ion site in the monazite structure can be easily substituted by any other lanthanide ions. To improve luminescent properties of nanocrystalline phosphors, many preparation methods have been used, such as solid state reactions, sol-gel techniques, hydroxide precipitation, hydrothermal synthesis, spray pyrolysis, laser-heated evaporation, and combustion synthesis[11-13]. 39

2 In this paper LaPO 4:Er 3+ (0.5 mol%) nanophosphor was prepared by the solid state reaction method in air at C, and their luminescent properties were studied. Optimization of the concentration of activator ions incorporated into the host lattice during the synthesis of the phosphor powders is essential for developing highly luminescent RE 3+ doped nanocrystalline phosphors as well as for the growth of grain particles. Photoluminescence studies and CIE co-ordinates of LaPO 4:Er 3+ nanophosphor reveals that the emission colour having excellent colour tunability of blue light so this material may be a potential luminescent material. 2.Experimental Synthesis: The LaPO 4:Er 3+ (0.5 mol %) nanophosphor was synthesized by using the conventional solid-state reaction method. The formation of the phosphor powder occurs according to the following chemical equation. La 2O (NH 4) H2PO4 + Er 2O 3 2LaPO 4: Er (0.5%) The starting materials were lanthanum oxide (La 2O 3), Diammonium Hydrogen Phosphate [(NH 4) 2 H 2 PO 4) and erbium oxide (Er 2O 3) of 99.9% purity. They were weighed with a certain stoichiometric ratio. The composite powders were grinded in an agate mortar and then placed in an alumina crucible with the lid closed. After the powders had been sintered at 1200 o C for 3 hr in a muffle furnace and then cooled to room temperature. All the samples were again ground into fine powder using an agate mortar and pestle about an hour. Finally, the powders were sieved again through 100 µm sieve. Characterization: Phase identification was carried out by X-ray diffraction (XRD) using a PAN alytical s X-ray diffractometers X'Pert PRO, operating at 50 kv and 30mA with a Cu Kα radiation, λ=0.154nm and a Ni filter, in the range 2θ = The step size was and the counting time 1.5 s.the morphology of the nanoparticles was observed by using a scanning electron microscope (TESCAN VEGA3 SEM) with a tungsten heated filament. The emission and the excitation spectra of the synthesized powders were characterized with a spectroflurophotometer(shimadzu RF 5301 PC) with xenon lamp as excitation source. The frequencies of the absorption bands were analyzed through Fourier transform infrared spectroscopy (Bruker Vector 22 FT-IR Spectrometer) for the as-prepared material. The pellets used for analysis were made of 0.01 g of the sample powders and 0.3 g of KBr.Infrared spectra for the prepared solid nano powders were recorded in the range between 400 and 4000 cm -1. The Commission International de l Eclairage (CIE) co-ordinates were calculated by the spectrophotometric method using the spectral energy distribution. The chromatic coordinates (x, y) of prepared materials were calculated with colour calculator version 2, software from Radiant Imaging [14]. 3. Results and Discussions Crystal structure of LaPO4:Er 3+ (0.5 mol %) nanophosphor: The crystal structure of the obtained product was identified by X-ray diffraction analysis (XRD). Fig.1 and 2 shows the typical X-ray diffraction (XRD) patterns of LaPO 4:Er 3+ (0.5 mol %) nanophosphor treated at C. The XRD spectra consist of three strong peaks and several weak peaks: The three main peaks occur at 2θ=26.82, and These peaks correspond to the diffractions from the (200),(120), and (012) planes of LaPO 4 :Er 3+ respectively. The relatively weak multipeaks centered at 21.17, 34.33,42 and are attributed to the diffraction from the (111), (202),(311), and (132) planes, respectively. The intensity of peaks reflected the high degree of crystallinity of the nanoparticles. All the diffraction peaks could be well indexed to International Centre for Diffraction Data (ICDD) database card number: , which indicated a base centered monoclinic phase LaPO 4:Er 3+ (space group P2 1/n) with a main diffraction peak (120).No spurious diffractions due to crystallographic impurities are found.the XRDA 3.1 software has been used to calculate the lattice parameters. Unit cell parameters values calculated from XRD are enumerated in the table-1 and observed that the cell volume of LaPO 4:Er 3+ (0.5 mol %) nanophosphor is slightly more than standard volume [15,16]. Table-2 shows the calculated crystallite size of LaPO 4:Er 3+ (0.5 mol %) nanophosphor. However, the diffraction peaks are broad which indicating that the crystalline size is very small. Intensity(arb.u) LaPO 4 : Er Figure.1 X-ray powder diffraction pattern of LaPO 4:Er 3+ (0.5 mol %) nanophosphor 40

3 Figure.2 X-ray powder diffraction pattern of LaPO 4:Er 3+ (0.5 mol %) nanophosphor Table 1 Crystallographic unit cell parameters LaPO 4 :Er 3+ (0.5 mol%) phosphor Lattice Parameters Sample a (nm) b(nm) c (nm) Bond angle(β)(d eg) Unit Cell volume(v) (nm 3 ) ICDD: Prepared Sample: LaPO 4phosphor LaPO 4:Er 3+. (0.5%) nanophosphor Table 2 The crystallite size of LaPO 4:Er 3+ (0.5 mol%) nanophosphor 2θ of intense peak(deg) (hkl) FWHM of intense peak (β radian) Crystallite size of particle (nm) The crystallite size was determined by means of the X-ray line broadening method using the Debye-Scherrer equation D= 0.9 λ / β cosθ, where λ is the wavelength of the X-ray (λ = 1.54A 0 for CuKα), β is the broadening of the diffraction line measured at half of its maximum intensity(fwhm:full width at half maximum), θ is the Bragg s diffraction angle and D is the crystallite size.the average crystallite size of LaPO 4:Er 3+ (0.5 mol%) nanophosphor C is 64.12nm. This confirms the formation of nano crystalline size phosphor, via solid state reaction method. Fig.3 (a,b) shows the crystal structure of LaPO 4 : Er 3+ (0.5 mol%) nanophosphor [17-20]. Fourier Transforms Infrared (FTIR) Spectroscopy analysis of LaPO4 : Er 3+ (0.5 mol %) nanophosphor In order to determine the chemical bonds in a molecule, FTIR analysis was carried out. Fig.4 shows the FTIR spectrum of LaPO 4 : Er 3+ (0.5 mol %) nanophosphor heated Figure. 3 (a,b) Crystal structure of (a) pure LaPO 4 and (b) LaPO 4: Er nanophosphor Figure 4 FTIR spectrograph of LaPO 4:Er 3+ (0.5 mol %) nanophosphor 41

4 PL Intensity(arb.u) Excitation spectrum Excitation spectrum monitored under 400nm 2. ex =254nm 2 451nm 469nm Emission spectrum Wave length(nm) 495nm 483nm475nm Fig.Excitation&Emission spectrum of LaPO 4 :Er 595nm 612nm CIE (1931-Chart) Coordinates of LaPO4:Er 3+ (0.5 mol%) nanophosphor Most lighting specifications refer to color in terms of the 1931 CIE chromatic colour coordinates which recognizes that the human visual system uses three primary colours: red, green, and blue. In general, the color of any light source can be represented on the (x, y) coordinate in this color space. The color purity was compared to the 1931 CIE Standard Source C (illuminant Cs (0.3101, )). The chromatic coordinates (x, y), was calculated using the color calculator program radiant imaging. Fig.6 shows the CIE co-ordinates of (chart -1931) of LaPO 4:Er 3+ (0.5 mol%) phosphor. The color co-ordinates for the LaPO 4:Er 3+ (0.5 mol%) phosphor are = and y= The location of the colour coordinates of the monophosphate. This phosphor is having excellent colour tunability of blue light. Figure 5 Excitation and emission spectra of LaPO 4:Er 3+ (0.5 mol%) nanophosphor at 1200 o C. Most of the bands are characteristic of the vibrations of phosphate groups PO 3-4 [21].It had been 3- reported that the PO 4 should have C 1 symmetry including two (v 3 and v 4) vibration regions in the monoclinic LaPO 4:Er 3+ (0.5 mol %) nanophosphor [22]. The v 3 vibration corresponds to the phosphate P- O stretching. The v 4 vibration corresponds to the O= P- O is bending and O- P- O is bending modes. So the characteristics of the monoclinic phase of four bands located at about 538,563,577,618 cm -1 were clearly observed in the ν 4 3- region (bending vibration) of PO 4 groups vibration. The characteristic bands at 952, 992, 1091 cm -1 belong to the v 3 vibration region. Split bands in the v3 region are characteristic of the monoclinic LaPO 4 :Er 3+ (0.5 mol%) phosphor phase[23].the vibration spectra give a conclusive evidence for monoclinic-phase formation in lanthanum phosphate.the peaks at 1271,1400,3134 cm -1 can be attributed to the presence of water (stretching vibration of the O-H bond)adsorbed by KBr during the pellet formation. Photoluminescence Study of LaPO4:Er 3+ (0.5 mol %) nanophosphor: Fig.5 shows PL excitation and emission spectra of LaPO 4: Er 3+ (0.5 mol%) nanophosphor recorded at room temperature. The excitation spectrum of LaPO 4 :Er 3+ (0.5 mol%) phosphor monitored under 400nm wavelength was characterized by a broad band ranging from nm with a maximum intensity peak at 246nm.The La 3+ ion site in the monazite structure can be easily substituted by any other lanthanide ions. The La 3+ ion site in the monazite structure can be easily substituted by any other lanthanide ions.the luminescent characteristics of the particles depend on its size and other properties including the degree of crystallization, defects and the valence state of the doped activator ions. The shape of the emission spectra and emission peak wavelength is independent of the excitation wavelengths.upon excitation at 254nm wavelength, the emission spectrum of LaPO 4:Er 3+ (0.5 mol %) nanophosphor emits a broad band range from nm with maximum intensity peak at 469(blue) nm. Figure 6 CIE Co-ordinates of LaPO 4:Er 3+ (0.5 mol%) Nanophosphor depicted on the 1931 chart 4. Conclusion LaPO 4:Er 3+ (0.5 mol %) nanophosphor powder were successfully synthesized through solid state reaction method at high temperature ( C) and the luminescent properties of sample was studied. The prepared LaPO 4:Er 3+ (0.5 mol %) nanophosphor in a single-phase,monoclinic structure with average crystallite size of 64.12nm. The width of diffraction peak is broadened because of the small size of the crystallites.the PL characterization demonstrates that the SLaPO 4:Er 3+ (0.5 mol%) nanophosphor shows the most intense emission.the Commission International de 1 Eclairage [CIE] coordinates of LaPO 4 :Er 3+ (0.5 mol%) nanophosphor exhibit the excellent colour tunability of blue.therefore, the LaPO 4 :Er 3+ (0.5 mol%) nanophosphor can be easily applied in various types of lamp and display due to its morphologies and good PL performance. Acknowledgements The author,niyaz Parvin Shaik, is gratefully thanking the University Grant Commission (UGC), New Delhi, India for 42

5 financial assistance under Maulana Azad National Fellowship for Minority students. References 1. Niinisto, L; Leskela, M; Gschneidner Jr. K. A; Eyring, L; (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Elsevier, Amsterdam,(1987) 2. Tuan, D. C; Olazcuaga, R; Guillen, F; Garcia, A; Moine, B; Fouassier, C; J. Phys. IV, 123, 259(2005) 3. Moine,B; Bizarri, G; Opt. Mater.,28,58(2006) 4. Blasse, G; Grabmaier, B.C; Luminescent Materials, Springer, Berlin,(1994) 5. Xu, L; Guo, G; Uy, D; E O Neill, A; Weber, W. H; Rokosz, M. J; McCabe,R.W; J. Appl. Catal. B, 50, 2, (2004). 6. Yan, R. X.; Xiaoming Sun.; Wang, X.; Peng, Q.; Li, Yadong.; Chemistry - A European Journal, 11,7, (2005) 7. Kang,Y.C.; Kim,E.J.; Lee,D.Y.; Park,H.D.; J.Alloys Compounds,347,1 2, (2002) 8. Meyssamy, H; Riwotzki, K; Kornowski, A; Naused, S; Haase; Adv.Mater.11,10, (1999) 9. Haase, M; Riwotzki, K; Meyssamy, H; Kornowski, A; J. Alloys Comp , (2000) 10. Andersson,J.;Areva,S.;Spliethoff,B.;Linden,M.;Biomat erials,26,34, (2005) 11. Chunxia Li.; Zewei Quan.; Jun Yang.; Yang,P.; Lin,J.; Inorg. Chem.46,16, (2007) 12. Fang, Y.P.; Xu, A.W.; Song, R.Q.; Zhang, H.X.; You, L.P.; Yu, J.C.; Liu, H.Q.; J. Am. Chem. Soc. 125, 51, (2003) 13. Atchyutha Rao, Ch; Poornachandra Rao. V. Nannapaneni; Murthy, K.V.R; Adv. Mat. Lett. 4(3), (2013) 14. Color Calculator version 2, software from Radiant Imaging, Inc, (2007), radiant imaging-colorcalculator.software.informer.com/2.0/, (2007) 15. Mullica, d.f., Milligan, W.O.,Grossie, D.A., Beall,G.W and Boatner,L.A Inorg. Chim. Acta. 95, (1984) 16. Wang Ruigang, Pan Wei, Chen Jian, Fang Minghao, Cao Zhenzhu, Luo Yongming, Materals Chemistry and Physics,79,30-36,(2003) 17. Satyajit Phadke, Juan C. Nino, M. Saiful Islam, J. Mater. Chem., 22, 25388(2012) 18. Bagatur yants,a.a,iskandarova,i.m, Knizhnik,A.A, Mironov,V.S, Potapkin,B.V, Srivastava,A.M, and Sommerer,T.J,Physical Review B 78, (2008) 19. Mei Yang, HongpengYou, Guang Jia, Yeju Huang, Yanhua Song, Yuhua Zheng, KaiLiu, Lihui Zhang, Journal of Crystal Growth 311, , (2009) 20. Mei Yang, Hongpeng You, Youlong Liang, Jingzhou Xu, Fan Lu,Liming Dai, Yong Liu, Journal of Alloys and Compounds 582, (2014) 21. Piaoping Yang, ZeweiQuan, ChunxiaLi, ZhiyaoHou, WenxinWang, Jun Lin, Journal of Solid State Chemistry,182, ,(2009) 22. Hezel, A, Ross, S. D, Spectrochimica Acta, 22, 1949, (1996) 23. Milligan, D. F, Grossie,W. O, Grossie, D. A, Beal, G. W, Boatner, L. A, Inorganica Chimica Acta, 95, 231,(1984) Source of support: Nil; Conflict of interest: None declared 43